One key element is the character interactions. Seeing characters like Clark Kent and Oliver Queen interact, given their different backgrounds and powers, is really exciting. Another element is the merging of storylines. For example, how the threats in Smallville might be related to those in the Arrowverse.
One idea could be a story where characters from different Arrowverse shows team up to fight a new, powerful villain from a different dimension. For example, Oliver Queen from 'Arrow', Barry Allen from 'The Flash', and Kara Danvers from 'Supergirl' could combine their skills and powers to take on this threat.
Maybe the Flash could visit the MCU. He could race against Quicksilver. It would be a really cool showdown of speedsters from different universes. And then the Flash could get involved in an MCU - level threat, like Thanos' invasion but in a different timeline. He could use his speed to try and steal the Infinity Gauntlet from Thanos, creating a whole new plotline for a fanfiction.
One exciting plot could be that the Infinity Stones' power somehow disrupts the multiverse and creates a rift that connects the Avengers' universe with the Arrowverse. The Avengers end up in Star City and team up with the Arrow team to fight a common enemy, like a powerful sorcerer who wants to use the chaos to take over both universes. They combine their skills, with the Avengers' superpowers and the Arrow team's combat and archery skills.
Hush could be an interesting choice. He could be spreading false rumors and causing distrust between the heroes and the citizens of both cities. His knowledge of Gotham's underworld and his ability to manipulate people could make him a formidable foe for Batwoman and Arrow. They would need to uncover his true identity and stop him before he completely destroys the relationship between the two cities.
A cool concept could be a crossover where the technology in Gotham and the magic in Star City somehow get mixed up. Batwoman, being more used to high - tech gadgets, and Arrow, who has some experience with magic - related threats, have to combine their skills to set things right. For example, a device that was supposed to enhance Batwoman's suit ends up being enchanted by magic from Star City, and they need to figure out how to fix it and stop the villain who caused the mix - up.
First, you need to have a good understanding of the different shows in the Arrowverse. Watch all the relevant seasons and take notes on the characters, their powers, and their relationships. Then, think of a unique premise for your crossover. It could be a new threat that requires characters from different shows to work together.
Well, it could be that there is a rift between dimensions that pulls the Arrowverse into the Willy Wonka universe. The characters from Arrowverse are confused at first, but then they realize they need to help Willy Wonka with a mystery in his factory. There could be some sort of dark magic involved that is affecting the production of the delicious treats. The Arrowverse characters would bring their own unique skills and personalities to solve the mystery, all while experiencing the wonder and strangeness of the Willy Wonka world.
One idea could be that the Winchesters end up in Star City and team up with the Arrow to fight a demon that is using technology to cause chaos. Maybe the demon has possessed some of the tech - savvy villains in the Arrowverse.
A third possibility is a crossover based on a multiverse - breaking event. Maybe a powerful magic user accidentally creates a rift between the Arrowverse and the Marvel universe. This leads to a chaotic situation where heroes and villains are randomly shuffled between the two universes. The heroes then have to figure out how to put things right while also dealing with the new threats they encounter in the foreign universes.